This invention relates to plastic draw tape trash bags and more particularly, to an improved hem seal for such a bag.
Bags made of thin polyethylene material have been used in various sizes. Small bags are used in the packaging of sandwiches and the like; larger bags are used as shopping bags and even larger bags are used for containing trash.
A particularly avantageous closure for such a bag includes a draw tape constructed from the same polyethylene material. U.S. Pat. No. 3,029,583--Piazzi, and British Pat. No. 1,125,363--Jortikka are examples of draw tape bags. Such closures have been successfully employed on moderate sized shopping bags. Draw tape closures have not been commercially successful on large trash bags.
The load in a trash bag may typically be as high as 30-50 lbs. With such a large load, the bag will tear at almost any weak point. Tearing is particularly troublesome if there is a weak line in the direction of orientation of the plastic. Bags are typically made from a continuous extruded tube of plastic. This plastic is oriented in the direction of extrusion. Heat seals made perpendicular to the direction of orientation usually present no problems. For example, the edges or sides of the bag are made by a heat seal across the extruded tube, i.e., perpendicular to the direction of orientation.
On the other hand, a heat seal which secures the draw tape in the hem of the bag, extends in the extruded direction across the width of the bag. Because this seal runs in the direction of orientation of the plastic, once a tear is started along a hem seal, it propagates rapidly. In one prior attempt to make a draw tape trash bag, an intermittent seal was used to secure the hem, possibly in an attempt to prevent propagation of tearing along the seal line. Instead of interrupting the tearing, the intermittent seal actually concentrated the stress along the tear line. The shoulders on either side of the intermittent seal were reoriented by the heat and pressure of the seal bar to form strong areas which concentrated the stress in the weak, heat and pressured thinned, area between them. Because of this, a tear, once started, propagated rapidly along the hem seal in this bag.
It is an object of the present invention to provide a draw tape trash bag which will withstand the heavy intended loads without tearing at the hem seal.